Video production switchers provide a function which layers multiple video sources. Each video source is associated with an alpha source which defines transparent areas of the video source on the top over lower layers. The base layer or the base source is called the background. Traditionally, the background is not associated with any alpha. It can be thought of as a fully visible layer which blocks any scenes behind it.
Conventional cascaded or sequential processing of video effects such as layering can entail significant delay.